A Facile Approach for the Fabrication of Highly Stable

Publication Date (Web): February 15, 2010 ... Carbon Nanotubes with Polymer Brushes and Its Application in Composite Hydrogels ..... Functionalization...
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A Facile Approach for the Fabrication of Highly Stable Superhydrophobic Cotton Fabric with Multi-Walled Carbon Nanotubes-Azide Polymer Composites Guang Li, Hu Wang, Haiting Zheng, and Ruke Bai* CAS Key Laboratory of Soft Matter Chemistry, Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei, 230026, People’s Republic of China Received November 15, 2009. Revised Manuscript Received February 2, 2010 Homogeneous dispersion and functionalization of pristine multiwalled carbon nanotubes (MWNTs) in various organic solvents was achieved by a simple ultrasonic process in the presence of an azide copolymer, poly(4-azidophenyl methacrylate-co-methyl acrylate)(P(APM-co-MA)). The copolymes were noncovalently attached to the surface of the MWNTs via π-π interactions to form MWNT-P(APM-co-MA) composites. The composites were characterized by transmission electron microscopy, thermogravimetric analysis, Raman spectra and UV-vis spectra. The solution dispersion of the MWNT-P(APM-co-MA) composites were used to prepare superhydrophobic cotton fabric by a facile dip-coating approach. MWNTs were covalently attached to the surface of the cotton fabric through the chemical reactions between the azide groups of P(APM-co-MA) with both MWNTs and cotton fibers. The reactions are based on UV-activated nitrene chemistry. Owing to the nanoscale roughness introduced by the attachment of MWNTs, the cotton fabric surface was transformed from hydrophilic to superhydrophobic with an apparent water contact angle of 154°. Since MWNTs were covalently attached on the surface of the cotton fabric, the superhydrophobicity possesses high stability and chemical durability.

Introduction Since their discovery in 1991,1 carbon nanotubes (CNTs) have attracted great attentions for their unprecedented physical, mechanical, electrical, thermostable and chemical properties,2-4 particularly in the fields of nanoscience and nanotechnology. However, their poor dispersibility and stability in both organic and aqueous solvents as a result of strong inter-CNT van der Waals interactions make them hard to be handled, which imposes a processing challenge for practical applications. In order to overcome such drawbacks of CNTs, various surface modification methods, which can be categorized as either covalent or noncovalent approaches, have been developed. The covalent approach5-15 has been investigated widely and proved to be an efficient strategy for the dispersion of CNTs. However, the *To whom correspondence should be addressed. E-mail: [email protected]. (1) Iijima, S. Nature 1991, 354, 56. (2) Dresselhaus, M. S.; Dresselhaus, G.; Avouris, P. In Carbon Nanotubes in Topics in Applied Physics; Springer: Berlin, 2001. (3) Rao, C. N. R.; Govindaraj, A. In Nanotubes and Nanowires; RSC Publishing: Cambridge, U.K., 2005. (4) Baughman, R. H.; Zakhidov, A. A.; de Heer, W. A. Science 2002, 297, 787. (5) Bahr, J. L.; Tour, J. M. Chem. Mater. 2001, 13, 3823. (6) Yao, Z.; Braidy, N.; Botton, G. A.; Adronov, A. J. Am. Chem. Soc. 2003, 125, 16015. (7) Liang, F.; Sadana, A. K.; Peera, A.; Chattopadhyay, J.; Gu, Z.; Hauge, R. H.; Billups, W. E. Nano Lett. 2004, 4, 1257. (8) Lou, X.; Detrembleur, C.; Pagnoulle, C.; Jer^ome, R.; Bocharova, V.; Kirity, A.; Stamm, M. Adv. Mater. 2004, 16, 2123. (9) Liu, Y.; Adronov, A. Macromolecules 2004, 37, 4755. (10) Li, H.; Cheng, F.; Duft, A. M.; Adronov, A. J. Am. Chem. Soc. 2005, 127, 14518. (11) Xu, H. X.; Wang, X. B.; Zhang, Y. F.; Liu, S. Y. Chem. Mater. 2006, 18, 2929. (12) Tasis, D.; Tagmatarchis, N.; Bianco, A.; Prato, M. Chem. Rev. 2006, 106, 1105. (13) Homenick, C. M.; Lawson, G.; Adronov, A. Polym. Rev. 2007, 47, 265. (14) Georgakilas, V.; Bourlinos, A. B.; Zboril, R.; Trapalis, C. Chem. Mater. 2008, 20, 2884. (15) Georgakilas, V.; Bourlinos, A.; Gournis, D.; Tsoufis, T.; Trapalis, C.; Mateo-Alonso, A.; Prato, M. J. Am. Chem. Soc. 2008, 130, 8733.

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structure and electronic properties of the CNTs were altered due to the chemical functionalization. The noncovalent approach has received significant attentions as it modifies CNTs without the introduction of defects. Noncovalent attachment of numerous aromatic species to CNTs surface has been widely reported, where the π-π interaction plays an important role in the dispersion of CNTs. Most notably, conjugated polymers such as poly(3-hexylthiophene),16-18 poly(phenylenevinylene),19-21 poly(aryleneethynylene),22,23 and poly(9,9-dialkylfluorene),24 as well as small aromatic molecules such as pyrene,25-33 porphyrin,34-40 and triphenylene41 derivatives that bind to CNTs through π-π (16) Goh, R. G. S.; Motta, N.; Bell, J. M.; Waclawik, E. R. Appl. Phys. Lett. 2006, 88, 053101. (17) Zou, J. H.; Khondaker, S. I.; Huo, Q.; Zhai, L. Adv. Funct. Mater. 2008, 18, 1. (18) Zou, J. H.; Liu, L. W.; Chen, H.; Khondaker, S. I.; McCullough, R. D.; Huo, Q.; Zhai, L. Adv. Mater. 2008, 20, 2055. (19) Curran, S. A.; Ajayan, P. M.; Blau, W. J.; Carroll, D. L.; Coleman, J. N.; Dalton, A. B.; Davey, A. P.; Drury, A.; McCarthy, B.; Maier, S.; Strevens, A. Adv. Mater. 1998, 10, 1091. (20) Dalton, A. B.; Stephan, C.; Coleman, J. N.; McCarthy, B.; Ajayan, P. M.; Lefrant, S.; Bernier, P.; Blau, W. J.; Byrne, H. J. J. Phys. Chem. B 2000, 104, 10012. (21) Star, A.; Stoddart, J. F.; Steuerman, D.; Diehl, M.; Boukai, A.; Wong, E. W.; Yang, X.; Chung, S. W.; Choi, H.; Heath, J. R. Angew. Chem., Int. Ed. 2001, 40, 1721. (22) Chen, J.; Liu, H.; Weimer, W. A.; Halls, M. D.; Waldeck, D. H.; Walker, G. C. J. Am. Chem. Soc. 2002, 124, 9034. (23) Rice, N. A.; Soper, K.; Zhou, N.; Merschrod, E.; Zhao, Y. Chem. Commun. 2006, 4937. (24) Cheng, F.; Imin, P.; Maunders, C.; Botton, G.; Adronov, A. Macromolecules 2008, 41, 2304. (25) Chen, R. J.; Zhang, Y.; Wang, D.; Dai, H. J. Am. Chem. Soc. 2001, 123, 3838. (26) Nakashima, N.; Tomonari, Y.; Murakami, H. Chem. Lett. 2002, 638. (27) Petrov, P.; Stassin, F.; Pagnoulle, C.; Jerome, R. Chem. Commun. 2003, 2904. (28) Liu, L.; Wang, T. X.; Li, J. X.; Guo, Z. X.; Dai, L. M.; Zhang, D. Q.; Zhu, D. B. Chem. Phys. Lett. 2003, 367, 747. (29) Lou, X.; Daussin, R.; Cuenot, S.; Duwez, A.; Pagnoulle, C.; Detrembleur, C.; Bailly, C.; Jer^ome, R. Chem. Mater. 2004, 16, 4005. (30) Georgakilas, V.; Tzitzios, V.; Gournis, D.; Petridis, D. Chem. Mater. 2005, 17, 1613. (31) Ou, Y. Y.; Huang, M. H. J. Phys. Chem. B 2006, 110, 2031.

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interactions have been used to disperse and functionalize CNTs successfully. On the other hand, superhydrophobic surfaces of textiles endow them with self-cleaning properties, and fabrication of super water-repellent textiles has been an attractive subject.42,43 Superhydrophobic surfaces with a water contact angle larger than 150° have been achieved mainly in two ways: one is to create surface roughness and the other is to lower the surface energy.44 It has been well demonstrated that surface roughness plays an important role in determining the wetting behavior of materials.42,43,45-47 Considering the unique physical properties of CNTs, the modification of textile material with CNTs would be of great importance for both fundamental research and practical applications. The assembly of CNTs on cotton fabric surface forms nanoscale surface roughness on microscale cotton fibers. Such micro-nanoscale binary structure leads to the formation of an artificial lotus leaf structure on the cotton fabric.48-53 There are only very few reports14,48,49 on the preparation of superhydrophobic cotton fabric with CNTs. In these reports, the MWNTs were chemically functionalized through oxidation followed by cycloaddition14 or polymerization.48,49 The modified MWNTs were then physically absorbed on the cotton fabric to form a superhydrophobic surface. However, since CNT-polymer composites physically absorbed on the surface of the cotton fabric are easily washed away by organic solvents, these approaches may not favor the durable application of the modified cotton fabric. It is well-known that azide group easily decomposes to nitrene under UV or thermal conditions. The nitrene has very high reactivity with almost any C-H bonds or CdC bonds of adjacent organic molecules. Therefore, azides are widely used as reactants for organic synthesis, as well as cross-linking agents for polymer materials.54-56Azide-photolysis and thermolysis have been used (32) Liu, Y.; Yu, Z. L.; Zhang, Y. M.; Guo, D. S.; Liu., Y. P. J. Am. Chem. Soc. 2008, 130, 10431. (33) Yang, Q.; Shuai, L.; Pan, X. J. Biomacromolecules 2008, 9, 3422. (34) Li, H. P.; Zhou, B.; Lin, Y.; Gu, L. R.; Wang, W.; Fernando, K. A. S.; Kumar, S.; Allard, L. F.; Sun, Y. P. J. Am. Chem. Soc. 2004, 126, 1014. (35) Chen, J. Y.; Collier, C. P. J. Phys. Chem. B 2005, 109, 7605. (36) Hasobe, T.; Fukuzumi, S.; Kamat, P. V. J. Am. Chem. Soc. 2005, 127, 11884. (37) Satake, A.; Miyajima, Y.; Kobuke, Y. Chem. Mater. 2005, 17, 716. (38) Guldi, D. M.; Taieb, H.; Rahman, G. M. A.; Tagmatarchis, N.; Prato, M. Adv. Mater. 2005, 17, 871. (39) Cheng, F.; Adronov, A. Chem.;Eur. J. 2006, 12, 5053. (40) Cheng, F.; Zhang, S.; Adronov, A. Chem.;Eur. J. 2006, 12, 6062. (41) Yang, L. P.; Pan, C. Y. Macromol. Chem. Phys. 2008, 209, 783. (42) Zhang, X.; Shi, F.; Niu, J.; Jiang, Y.; Wang, Z. J. Mater. Chem. 2008, 18, 621. (43) Roach, P.; Shirtcliffe, N. J.; Newton, M. I. Soft Matter 2008, 4, 224. (44) Feng, L.; Li, S.; Li, H.; Zhai, J.; Song, Y.; Jiang, L.; Zhu, D. Angew. Chem., Int. Ed. 2002, 41, 1221. (45) Shibuichi, S.; Onda, T.; Satoh, N.; Tsujii, K. J. Phys. Chem. 1996, 100, 19512. (46) Patankar, N. A. Langmuir 2004, 20, 7097. (47) Bormashenko, E.; Stein, T.; Whyman, G.; Bormashenko, Y.; Pogreb, R. Langmuir 2006, 22, 9982. (48) Liu, Y.; Wang, R.; Lu, H.; Li, L.; Kong, Y.; Qi, K.; Xin, J. H. J. Mater. Chem. 2007, 17, 1071. (49) Liu, Y.; Wang, X.; Qi, K.; Xin, J. H. J. Mater. Chem. 2008, 18, 3454. (50) Wang, T.; Hu, X.; Dong, S. Chem. Commun. 2007, 1849. (51) Hoefnagels, H. F.; Wu, D.; de With, G.; Ming, W. Langmuir 2007, 23, 13158. (52) Leng, B.; Shao, Z.; de With, G.; Ming, W. Langmuir 2009, 25, 2456. (53) Gao, Q.; Zhu, Q.; Guo, Y.; Yang, C. Q. Ind. Eng. Chem. Res. 2009, 48, 9797. (54) Ruud, C. J.; Jia, J. P.; Baker, G. L. Macromolecules 2000, 33, 8184. (55) Varma, I. K. Macromol. Symp. 2004, 210, 121. (56) Akhrass, S. A.; Ostaci, R. V.; Grohens, Y.; Drockenmuller, E.; Reiter, G. Langmuir 2008, 24, 1884. (57) Holzinger, M.; Vostrowsky, O.; Hirsch, A.; Hennrich, F.; Kappes, M.; Weiss, R.; Jellen, F. Angew. Chem., Int. Ed. 2001, 40, 4002. (58) Holzinger, M.; Abraham, J.; Whelan, P.; Graupner, R.; Ley, L.; Hennrich, F.; Kappes, M.; Hirsch, A. J. Am. Chem. Soc. 2003, 125, 8566.

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successfully to functionalize CNTs,57-64 where azide groups link CNTs and polymers covalently without the use of strong acid treatments that inclines to damage the intrinsic structure of CNTs. Meanwhile, multistep reactions on the surface of CNTs are not needed. Compared with the thermolysis process requiring high temperature and longer reaction time,58,60,61,64 the UV photolysis is more facile because it is conducted at ambient temperature with a high reaction rate.59,62,63 Modification of fibers and films through covalently attaching CNTs to their surfaces have not been reported yet. In this contribution, aromatic azide copolymer, poly(4-azidophenyl methacrylate-co-methyl acrylate) (P(APM-co-MA)), is utilized successfully to disperse MWNTs in various organic solvents through π-π interactions to form MWNT-P(APMco-MA) composites. Stable superhydrophobic cotton fabric is successfully prepared by dip-coating the cotton fabric with MWNT-P(APM-co-MA) composites, which is then followed by UV irradiation. In comparison with the previous works, our approach has unique advantages. Azide copolymers was used as a linking agent to covalently link MWNTs to the cotton fabric surface in one step and the photochemical reaction was easily performed at room temperature in several minutes. Prefunctionalization of MWNTs via oxidation or other reactions was not needed. In addition, the superhydrophobicity of the covalently modified cotton fabric was highly stable for acids, bases, and organic chemicals. Our approach provides a very useful strategy for the preparation of highly durable and robust superhydrophobic fabrics.

Experimental Section Materials. MWNTs were provided by Tsinghua-Nafine Nano-Powder Commercialization Engineering Centre and used as received. P(APM-co-MA) was synthesized according to previously published procedures.65 Gel permeation chromatography (GPC) measurement using polystyrene as standard indicated a number-average molecular mass Mn of 16400 g/mol with a polydispersity index Mw/Mn = 1.17. The composition of P(APM-co-MA) was estimated to contain 23.9 and 76.1 mol % of APM and MA, respectively, according to 1H NMR spectral analysis. Cotton fabric was purchased from a general store. All other chemicals were analytical-grade reagents and used as received. Dispersion of MWNTs in Organic Solvents. Dispersions were prepared by sonication of 4.3 mg of MWNTs in 5 mL of 2.1 mg/mL P(APM-co-MA) organic solution for 20 min. Centrifugation (4000 rpm, 30 min, room temperature) of the sample was followed by decantation of the supernatant from above the precipitate. Preparation of the Superhydrophobic Cotton Fabric. The cotton fabric was cleaned by ultrasonic washing in ethanol and water, respectively, and then dried at 120 °C in a vacuum oven for 1 h. The cleaned fabric was allowed to soak in the dispersion of MWNTs in tetrahydrofuran (THF) for 1 min and dried at 80 °C for 10 min to remove the solvent. UV irradiation was carried out by placing the hand-held UV lamp (254 nm, 6 W) directly over the substrate at a distance of 5 cm for 10 min (the minimum (59) Moghaddam, M. J.; Taylor, S.; Gao, M.; Huang, S.; Dai, L.; McCall, M. J. Nano Lett. 2004, 4, 89. (60) Qin, S.; Qin, D.; Ford, W. T.; D. Resasco, E.; Herrera, J. E. Macromolecules 2004, 37, 752. (61) Holzinger, M.; Steinmetz, J.; Samaille, D.; Glerup, M.; Paillet, M.; Bernier, P.; Ley, L.; Graupner, R. Carbon 2004, 42, 941. (62) Lee, K. M.; Li, L.; Dai, L. J. Am. Chem. Soc. 2005, 127, 4122. (63) Pastine, S. J.; Okawa, D.; Kessler, B.; Rolandi, M.; Llorente, M.; Zettl, A.; Frechet, J. M. J. J. Am. Chem. Soc. 2008, 130, 4238. (64) Gao, C.; He, H.; Zhou, L.; Zheng, X.; Zhang, Y. Chem. Mater. 2009, 21, 360. (65) Li, G.; Zheng, H. T.; Bai, R. K. Macromol. Rapid Commun. 2009, 30, 442.

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Article Scheme 1. Schematic Illustration of Dispersing MWNTs by P(APM-co-MA)

irradiation time for successful modification was not determined). Then the substrate was rinsed with abundant THF and dried at 80 °C for 20 min. Characterizations. The morphologies of the dispersed MWNTs were characterized by transmission electron microscopy (TEM) using a JEOL-2010 transmission electron microscope operated at 200 kV. The samples were prepared by placing a droplet dispersion of MWNT in toluene onto a carbon-coated copper grid followed by air-drying. Thermal gravimetric analysis (TGA) was carried out on a PE TGA-7 instrument at a heating rate of 10 °C 3 min-1 under nitrogen. Raman spectroscopic analyses were carried out on a LABRAM-HR confocal laser microRaman spectrometer at room temperature. The UV-vis absorption spectra were recorded on a UV-2409PC instrument. Fourier transform infrared (FT-IR) spectra were recorded on a Bruker Vector-22 IR spectrometer using KBr pellets. Apparent water contact angles were measured with an optical contact angle meter (Solon (Shanghai) technology science Co. Ltd.) at ambient temperature. Water droplets were dropped carefully onto the samples surfaces, and the average value of five measurements at different positions of the sample was adopted as the apparent contact angle. Advancing and receding contact angles were measured by adding and withdrawing water using a syringe. The time dependence of the apparent water contact angle measurement was conducted at a relative atmosphere humidity of 85% at 20 oC. X-ray photoelectron spectroscopy (XPS) analysis was performed on an ESCA Lab MKII instrument with Al KR radiation as the exciting source. Scanning electron microscopy (SEM) images were recorded using a JSM-6700F field-emission microscope. Atomic force microscopy (AFM) images were recorded under ambient conditions using a Digital Instrument Multimode Nanoscope IIIa operating in the tapping mode regime.

Result and Discussion P(APM-co-MA) was synthesized through room temperature RAFT polymerization using a traditional redox initiator (benzoyl peroxide/N,N-dimethyl aniline) as reported in our previous work.65 This aromatic azide polymer contains 23.9 mol % of phenyl azide groups, according to 1H NMR spectral analysis. Dispersion of MWNTs was conducted by ultrasonicating MWNTs and P(APM-co-MA) in different solvents (Scheme 1). We noticed that the pristine MWNTs dispersed in organic solvents by ultrasonication precipitated quickly afterward, but those dispersed in P(APM-co-MA) solutions were very stable. No sedimentation was observed even after storing the dispersion under dark conditions for 10 months. This result indicates that P(APM-co-MA) is an efficient dispersant for MWNTs. Figure S1 (Supporting Information) shows the images of the ink-like macroscopically homogeneous dispersions of MWNTs in THF, chloroform, dimethylformamide (DMF) and toluene. The microstructure of the dispersion was investigated using TEM. Figure 1(a) shows an overview of the dried dispersion and Langmuir 2010, 26(10), 7529–7534

Figure 1. TEM images of (a) MWNTs dispersed by P(APMco-MA) and (b) a single MWNT modified by P(APM-co-MA).

Figure 2. TGA curves of (a) the pristine MWNTs, (b) the MWNT-P(APM-co-MA) composites, and (c) P(APM-co-MA). Inset is the enlarged TGA curve of the MWNT-P(APM-co-MA) composites.

it can be seen that the majority of MWNTs are debundled. The TEM image of a single MWNT segment (Figure 1b) reveals the presence of amorphous coating on the surface of carbon nanotube. In order to investigate the amount of adsorbed polymer chains, the dispersion of MWNTs in THF was vacuum-filtrated through a 0.2 μm PTFE membrane, and washed with excess THF to remove the unbound polymers. The collected black solids were dried in a vacuum oven overnight at room temperature and investigated by TGA. Figure 2 shows the TGA curves of pristine MWNTs, MWNT-P(APM-co-MA) composites, and P(APMco-MA) samples. It is seen clearly that no distinct weight loss occurs in trace a, i.e., the pristine MWNTs remain stable below 700 °C. In contrast, 90% weight loss of P(APM-co-MA) is DOI: 10.1021/la904337z

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Figure 3. UV-vis absorption spectra of (a) P(APM-co-MA) and (b) the MWNT-P(APM-co-MA) composites.

observed in the same temperature range (trace c), resulting from thermal decomposition of the P(APM-co-MA) sample. The TGA measurement reveals that P(APM-co-MA) decomposes in two main steps corresponding to the loss of azide groups and the main chains of the polymer, respectively. As can be seen from trace b as well as the inset figure at a different scale, the MWNT-P(APMco-MA) composites also show the two decomposition steps resembling the weight loss steps of the neat polymers. The weight loss of the MWNT-P(APM-co-MA) composites is thus undoubtedly assigned to the thermal decomposition of P(APMco-MA). On the basis of these TGA traces, the weight fraction of the polymers attached to MWNTs is calculated to be about 17.5%. To further examine the presence of the polymer coating layers on MWNTs, we characterized the composites using a Raman spectroscope. The Raman spectra of the pristine MWNTs and MWNT-P(APM-co-MA) composites in Figure S2 (Supporting Information) reveal two characteristic bands of MWNTs at around 1350 (D-band) and 1580 cm-1 (G-band), which are attributed to the disorder transition mode peaks and characteristic tangential stretch mode peaks.66 The G-band is an intrinsic feature of CNTs that is closely related to vibrations in all sp2 carbon materials. In the spectra of the pristine MWNTs, the G-band is at 1585 cm-1, but in the presence of polymer, this band shifts to 1587 cm-1. The attachment of the polymer to MWNTs results in the observed upshift of G-band peaks due to the increased elastic constant of the harmonic oscillator of the polymer-coated MWNTs. For the dispersed MWNTs, a sharp increase in the D-band intensity as well as the ratio of the D-band to that of the G-band intensity should be noted. The attachment of P(APM-co-MA) to the surface of MWNTs causes the field disturbance and the physical strain in the graphite skeleton and then result in the augmentation of the D-band intensity.67 In order to understand the interactions between the polymers and the MWNTs, UV-vis absorption spectra were measured at room temperature in dilute THF solution. The UV-vis absorption spectrum of free P(APM-co-MA) reveals three major vibronic bands at 256, 262, and 269 nm (Figure 3a). Owing to the attachment of polymers to the MWNTs, the characteristic (66) Jorio, A.; M. Pimenta, A.; Souza Filho, A. G.; Saito, R.; Dresselhaus, G.; Dresselhaus, M. S. New J. Phys. 2003, 5, 139.1. (67) Sinani, V. A.; Gheith, M. K.; Yaroslavov, A. A.; Rakhnyanskaya, A. A.; Sun, K.; Mamedov, A. A.; Wicksted, J. P.; Kotov, N. A. J. Am. Chem. Soc. 2005, 127, 3463.

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Figure 4. XPS spectra of (a) the pristine MWNTs and (b) the MWNT-P(APM-co-MA) composites after UV irradiation.

absorption bands of the polymer with considerably lower absorbance values are broadened and no longer clearly distinguished, and a blue shift is also observed (Figure 3b). The band broadening and blue-shift may be due to the π-π stacking interaction, resulting in delocalization of π electrons onto the MWNT surface.21,41 Because of the presence of MWNTs, the absorptions are superimposed on a remarkable rising background in the spectrum of the MWNT-P(APM-co-MA) composites.30 It is well-known that aryl azide group can be photochemically activated to lose nitrogen and form nitrene group. This nitrene group has a high tendency to react with almost any C-H bonds or CdC bonds of an adjacent organic molecule.54-56 Figure S3 (Supporting Information) shows the FT-IR spectra of the P(APM-co-MA) film before and after UV exposure for different periods of time, presenting the spectral changes due to decomposition of the azide group. The characteristic band of azide group located at 2116 cm-1 decreases gradually upon UV exposure, which indicates that the decomposition of the azide groups takes place to give rise to the nitrenes. It has been reported that nitrenes can react with the side wall of the CNTs.57-64 In our research system, we consider that the reactions of nitrenes occur simultaneously with both CdC bonds of the MWNTs and C-H bonds of cellulose in the cotton fabric when the modified cotton fabric is irradiated under UV light. With this approach, the MWNTs are covalently attached to the surface of the cotton fabric. X-ray photoelectron spectroscopy (XPS) analysis is a powerful tool for the characterization of the CNTs before and after functionalization.11,58,62 The solid composites of MWNTP(APM-co-MA) were obtained by vacuum-filtration of the dispersion through a 0.2 μm PTFE membrane, washed with excess THF to remove the unbound polymers, and then irradiated by UV. As can be seen from Figure 4(a), the XPS of the pristine MWNTs shows the predominant C 1s peak at 284 eV and a weak O 1s peak at 532 eV (O/C = 0.015) due to the absorbed oxygen.62 After modification, the strong O 1s peak (O/C=0.26) is observed, and a N 1s peak at 400 eV appears (Figure 4b). The highresolution N 1s spectrum of the modified MWNTs (Figure S5 (Supporting Information)) clearly shows the peak of N 1s, which is attributed to the nitrogen atom of the aziridine groups from the reaction of the azide group with CNTs,58,62 indicating the formation of the covalent bonding between MWNTs and the copolymer. The appearance of the new peak at 289 eV in the Langmuir 2010, 26(10), 7529–7534

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high-resolution C 1s spectrum of the modified MWNTs (Figure S5 (Supporting Information)) is ascribed to the CdO bond of the copolymer. On the basis of the XPS analysis, it is concluded that the copolymer are covalently attached to the surface of the MWNTs. The surface wettability of the modified cotton fabric is assessed by apparent water contact angle (WCA) measurement. Owing to the surface roughness of the modified cotton fabric, the notion of “apparent water contact angle” is used, which is different from the notion of the “intrinsic contact angle” for the WCA measurement of the flat surface. The water is absorbed rapidly to the unmodified cotton fabric due to its perfect hydrophilic character. However, after modification with MWNT-P(APM-co-MA) composites, the cotton fabric is transformed from hydrophilicity

Figure 5. Image showing the self-cleaning ability of a dusted modified cotton fabric surface.

Article

to superhydrophobicity. At first, a 2 μL water droplet was used on the modified surface to measure apparent WCA. Interestingly, the water droplet did not stick on the surface because of the superhydrophobic property. Consequently, the volume of the water droplet was increased to 4 μL and the apparent WCA was measured to be 154° (Figure S4a (Supporting Information)). Apparently, introducing MWNTs to the surface of the cotton fabric generates nanoscale roughness on the surface of microscale cotton fiber,48,49 and the combination of micro- and nanostructures endows the surface of the materials with excellent superhydrophobicity.48-53 In order to further prove this, a controlled experiment was performed. A cotton fabric was modified by P(APM-co-MA) alone under the same conditions without MWNTs. In this case, a 2 μL water droplet was easily placed on the modified surface, and the apparent WCA was only 132° (Figure S4c (Supporting Information)). So we conclude that the MWNTs play an important role in forming superhydrophobic surface. It should be pointed out that UV irradiation is necessary for the covalent modification of cotton fabric with the MWNT-P(APM-co-MA) composites, the result of a controlled experiment on modified cotton fabric under the same conditions but without UV irradiation showed that the water droplet was absorbed immediately since the MWNT-P(APM-co-MA) composites were rinsed away with THF (Figure S4b (Supporting Information)). The advancing contact angle (θA) and receding contact angle (θR) of the modified cotton fabric were measured to be 155° and 152° respectively. The small contact angle hysteresis (θA - θR= 3°) of the modified cotton fabric was demonstrated by the free movement of water droplets even when the surface was only slightly tilted by 4°. Figure 5 shows a water droplet rolling over

Figure 6. SEM images of (a) original cotton fabric and (b) modified cotton fabric. AFM images of (c) original cotton fabric and (d) modified cotton fabric. Langmuir 2010, 26(10), 7529–7534

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a dusted modified cotton fabric and the removal of the dust along the path of the rolled droplet, indicating that the surface has selfcleaning effect similar to the lotus leaf. The time dependence of the apparent water contact angle for the modified cotton fabric is shown in Figure S6 (Supporting Information), and the contact angle of water droplet remains constant even after resting on the surface for 1 h. Furthermore, we found that the modified cotton fabric maintained the essentially constant apparent WCA value after 1 month storage in air, indicating that the superhydrophobic cotton fabric has good long-term stability. The surface morphologies of the original and modified cotton fabric were investigated by SEM and AFM (Figure 6). Parts a and c of Figure 6 indicate that the original cotton fabric presents a highly textured microscale fiber with a typically smooth surface. The SEM image (Figure 6b) shows the typical top view of the modified cotton fabric, and it can be seen that a thin layer with the nanoscale linear protuberances are armored on the as-prepared superhydrophobic fiber surfaces. The AFM image of the modified cotton fabric (Figure 6d) also reveals the rough surface with height variations caused by the random deposition of the MWNT-P(APM-co-MA) composites, and the dispersed MWNTs with the linear protuberances marked by the arrows can be clearly seen. This observation indicates that the MWNT-P(APM-co-MA) composites are well deposited on the surface of cotton fibers, which prevent the contact of water with hydroxyl groups on the fiber surface, resulting in the formation of the superhydrophobic cotton fabric. This is in good agreement with the apparent WCA measurements of the modified cotton fabric. For practical applications, the chemical durability of the superhydrophobic surface is a key issue, therefore the chemical resistance of the modified cotton fabric has been evaluated by measuring the change of apparent WCA values after being treated with aqueous solutions of varying pH (Figure 7) according to the methods described previously by Li et al.68,69 It is observed that the superhydrophobic cotton fabric displays a high durability with apparent WCA values >145° after treatment with acidic, neutral and basic solutions for 96 h. Though the treatment of the strongly basic solution (pH = 12) causes the apparent WCA values of the modified cotton fabric to decrease, the chemical resistance is still much higher in comparison with other modification methods.68,69 It is concluded that the presence of MWNTs endows the cotton fabric not only with superhydrophobicity, but also with the erosion resistance.

Conclusion In summary, the aromatic azide copolymer, P(APM-co-MA), has been successfully used to disperse and functionalize MWNTs (68) Li, S. H.; Xie, H. B.; Zhang, S. B.; Wang, X. H. Chem. Commun. 2007, 4857. (69) Li, S. H.; Zhang, S. B.; Wang, X. H. Langmuir 2008, 24, 5585.

7534 DOI: 10.1021/la904337z

Li et al.

Figure 7. Relationship between apparent water contact angles and immersion time at varying pH for the superhydrophobic cotton fabric.

in various organic solvents by a simple ultrasonic process. The composites of MWNTs with P(APM-co-MA) are formed in organic solvents through the noncovalent π-π interactions, and the MWNT dispersions are quite stable even after 10 months. Most importantly, the functional azide groups can be introduced into the dispersed MWNTs composites for further applications. A facile, high efficient, and one-step approach to transform cotton fabric from hydrophilic to superhydrophobic has been developed by dip-coating the fabric with the MWCNT-azide polymer composites and UV irradiation, without the use of multistep chemical reactions to prefunctionalize CNTs. In addition, the superhydrophobicity is highly stable for acids, bases, and organic chemicals treatments owing to the covalent attachment of MWNTs to the surface of cotton fabric. The creation of superhydrophobic, self-cleaning cotton fabric has potential applications in the textile industry, and we expect this facile and effective strategy will become a powerful platform for the fabrication of highly stable superhydrophobic materials with carbon nanotubes. Acknowledgment. The financial support from the National Natural Science Foundation (NNSF) of China (No. 20674076) and Ministry of Science and Technology of China (NO. 2007CB936401) is gratefully acknowledged. Supporting Information Available: Figures showing dispersions of MWNTs in P(APM-co-MA) solution, Raman and FT-IR spectra, photos of a water droplet on the cotton fabric, high-resolution F 1s and N 1s XPS spectra, and a plot of contact angle as a function of time. This material is available free of charge via the Internet at http://pubs.acs.org.

Langmuir 2010, 26(10), 7529–7534